1. Field of the Invention
The present invention relates to a light receiving circuit.
2. Description of Related Art
At present, photocouplers are widely used. Each photocoupler is used in a device including a drive unit operating at high voltage for industrial and consumer use in order to ensure electrical isolation between the drive unit operating at high voltage and a control unit operating at low voltage. The photocoupler transmits an electrical input signal to an output side by turning on/off light, and outputs the light as an electrical signal again.
A general-purpose photocoupler is required to be low in price and high in reliability. A circuit configuration in which such a photocoupler can be used as a light receiving circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2004-328061. FIG. 5 shows a circuit diagram of a light receiving circuit 1 of a prior art disclosed in Japanese Unexamined Patent Application Publication No. 2004-328061. As shown in FIG. 5, the light receiving circuit 1 includes a photodiode PD1, a capacitor element C1, NPN transistors Tr11, Tr12, Tr21, and Tr22, and resistor elements R11 to R13 and R21 to R23.
Additionally, FIG. 6 shows an integrated circuit (hereinafter referred to as “light receiving circuit 2”) of a related art, which employs the configuration of the light receiving circuit 1. As shown in FIG. 6, the light receiving circuit 2 includes the photodiode PD1, the NPN transistors Tr11, Tr12, Tr21, and Tr22, NPN transistors Tr31, Tr32, Tr41, Tr42, and Tr51, the resistor elements R11 to R13 and R21 to R23, and resistor elements R31 to R33, R41 to R43, and R51 to R53. Components of FIG. 6 denoted by the same reference symbols as those in FIG. 5 have the same configuration. The NPN transistors Tr11 and Tr12 and the resistor elements R11 to R13 constitute an amplifier AMP11. The NPN transistors Tr21 and Tr22 and the resistor elements R21 to R23 constitute an amplifier AMP21. The NPN transistors Tr31 and Tr32 and the resistor elements R31 to R33 constitute an amplifier AMP31. The NPN transistors Tr41 and Tr42 and the resistor elements R41 to R43 constitute an amplifier AMP41.
FIG. 7 is a block diagram showing a photocoupler 10 incorporating the light receiving circuit 2 shown in FIG. 6. The photocoupler 10 includes the light receiving circuit 2, input terminals IN1 and IN2, and a light emitting device LED1. The light emitting device LED1 is composed of a light emitting diode or the like. Components of FIG. 7 denoted by the same reference symbols as those in FIG. 6 have the same configuration.
The amplifier AMP11 is a transimpedance amplifier (current-voltage conversion amplifier) for shaping the waveform of an input signal of the amplifier AMP21 to improve a delay of an output signal of the light receiving circuit 2. An output signal of the amplifier AMP11 is input to the amplifier AMP21 through the capacitor element C1. The amplifier AMP21 is a transimpedance amplifier that converts a photocurrent Ipd, which is generated by the photodiode PD1, into a voltage signal. As seen from FIG. 6, the amplifiers AMP11 and AMP21 have the same circuit configuration. The input terminals of the amplifiers AMP11 and AMP21 are each connected to the photodiode PD1. In this configuration, even when a supply voltage of a power supply voltage VDD fluctuates, the amplifiers AMP11 and AMP21 operate in a similar manner with respect to the fluctuation. For this reason, this configuration has the characteristic of high resistance to noise.
When the photocurrent Ipd is equal to or smaller or larger than a predetermined current value, a bias current is supplied to an input transistor of the amplifier AMP21 so as to invert the phase of an output level of the amplifier AMP21. This function enables rapid inversion of the phase of an output voltage signal Vout which is described later, when the photocurrent Ipd reaches the predetermined current value. Thus, the amplifier AMP31 is an amplifier necessary for adjusting the sensitivity of the photocoupler 10. Also the amplifier AMP31 has the same circuit configuration as that of the amplifiers AMP11 and AMP21 so that the resistance to power supply noise is increased. A current I31 output from the amplifier AMP31 flows through the resistor element R53. A current I21 is a base current of the NPN transistor Tr21 which serves as the input transistor of the amplifier AMP21. The current I21 herein described is equal to a sum of the photocurrent Ipd and the current I31. When the current I31 is caused to flow from the amplifier AMP31, the sensitivity of the light receiving circuit 2 can be improved compared to the case where only the photocurrent Ipd is caused to flow. Moreover, setting of the sensitivity can be changed by adjusting a resistance value of the resistor element R52.
The amplifier AMP41 amplifies a signal output from the amplifier AMP21 and outputs the amplified signal. Then, the output signal drives the NPN transistor Tr51 which serves as an output transistor. The NPN transistor Tr51 is connected in an open collector configuration.
The operation of the photocoupler 10 will be described below with reference to FIGS. 8A to 8G. It is assumed that an output terminal OUT is applied with a bias voltage of 5V. FIGS. 8A and 8B show current waveforms and FIGS. 8C to 8G show voltage waveforms.
First, an input current signal Iin as shown in FIG. 8A is input to each of the input terminals IN1 and IN2. In response to the input current signal Iin, the light emitting device LED1 emits light. The light-receiving element PD1 receives the light signal from the light emitting device LED 1. In response to the received light signal, the light-receiving element PD1 outputs the photocurrent Ipd.
The amplifier AMP21 receives a voltage signal as shown in FIG. 8C. The voltage signal is generated from the photocurrent Ipd at a node B. The base of the input transistor (NPN transistor Tr21) of the amplifier AMP21 receives a current having a waveform shown in FIG. 8B which is described later.
The amplifier AMP11 converts a photocurrent, which is generated by a parasitic diode of the photodiode PD1, into a voltage signal as shown in FIG. 8D, and outputs the voltage signal to a node A. The voltage signal is transmitted to the node B through the capacitor element C1. Thus, the current injected into the base of the input transistor of the amplifier AMP21 has a waveform as shown in FIG. 8B. In this manner, the output waveform of the amplifier AMP11 is transmitted to the node B through the capacitor element C1, thereby shaping the waveform of the current signal input to the amplifier AMP21. Further, the rise time and fall time of a signal shown in FIG. 8E, which is output to a node C by the amplifier AMP21, is shortened. This is effective for providing an improvement in propagation delay time of the photocoupler 10.
The amplifier AMP41 receives the signal output to the node C by the amplifier AMP21. The amplifier AMP41 then amplifies the output signal of the amplifier AMP21, and outputs a signal having a waveform as shown in FIG. 8F to a node D.
The NPN transistor Tr51 is driven by a voltage signal output to the node D by the amplifier AMP41. When the NPN transistor Tr51 is in an ON state, the output voltage signal Vout at the output terminal OUT is at a low level. When the NPN transistor Tr51 is in an OFF state, the output voltage signal Vout is at a high level. Accordingly, the output voltage signal Vout is a voltage signal having a waveform as shown in FIG. 8G.
In a general-purpose photocoupler required to be low in price and high in reliability, the photocoupler 10 has a major advantage of requiring a small number of circuit elements, having high resistance to power supply noise, and capable of high-speed operation.
FIG. 9 shows a relationship between the photocurrent Ipd which is the input signal of the light receiving circuit 2 of the photocoupler 10 and the output voltage signal Vout as described above. As shown in FIG. 9, when the photocurrent Ipd gradually increases and reaches a predetermined value P, the output voltage signal Vout falls. On the contrary, when the photocurrent Ipd gradually decreases and reaches the predetermined value P, the output voltage signal Vout rises. A point at which the current value of the photocurrent Ipd reaches the predetermined value P is hereinafter referred to as “sensitivity point”. Thus, in the light receiving circuit 2, the output voltage signal Vout rises or falls at the time when the current value of the photocurrent Ipd, which is the input signal, reaches the sensitivity point. That is, the light receiving circuit 2 has no hysteresis characteristics with respect to input and output signals.